[llvm] DAG: Consider __sincos_stret when deciding to form fsincos (PR #165169)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> This seems reasonable. Are you planning on doing anything for AArch64 (which has a similar pattern, but falls back to `Expand`)?

Yes, AArch64 should do the same. It happens AArch64 seems to have better test coverage, and I see a regression in the v3 case where the stack slot isn't reused for each element or something 

https://github.com/llvm/llvm-project/pull/165169